The treatment of black liquor for recovery of pulping chemicals and heat value is an important and often limiting step in the kraft pulping process. Black liquor is a complex mixture of organic wood derivatives and alkaline pulping chemicals, chiefly containing degraded lignin, organic acid salts, resins, sodium hydroxide, and sodium salts including carbonate, sulfide, sulfate, sulfite, thiosulfate, and mercaptide. Weak black liquor typically contains 15 wt % dissolved and suspended solids of which about 80% are organic compounds and the remainder are inorganic compounds.
Weak black liquor is concentrated to about 45-50 wt % solids by multiple effect evaporation and further evaporated by direct contact evaporation to about 65 wt % solids. The concentrated liquor is combusted in a recovery boiler to raise steam and to recover sulfur and sodium for reuse in the pulping step. Oxidation of the sodium sulfide in the black liquor is necessary prior to the direct contact evaporator to minimize the emissions of hydrogen sulfide in the flue gas from the recovery boiler. Newer mills have replaced direct contact evaporators with indirectly-heated concentrators which eliminate total reduced sulfur emissions and yield higher liquor solids concentrations of up to 75 wt % before firing to the recovery boiler.
The capacity of the recovery boiler to combust black liquor and recover inorganic pulping chemicals often limits the production capacity of a pulp mill. The maximum capacity of the recovery boiler is typically limited by one or more parameters including fireside deposit formation, fume formation, and the maximum steam production rating of the boiler. Fireside deposit formation is caused by high temperatures in the pendant heat transfer surfaces of the boiler and high gas velocities from the furnace of the recovery boiler. Fume formation is caused by high temperatures in the furnace. The steam production rate at a constant firing rate of black liquor is a function of the available heat of the black liquor.
Increasing the solids concentration of the black liquor fired to the recovery boiler has several effects on the boiler operation. First, the fireside deposit formation rate is decreased due to decreased temperature and gas velocity of the gas from the furnace. At the same time, fume formation is increased by an increase in the temperature in the lower section of the furnace. Further, the steam production rate is increased due to the increased available heat of the black liquor. However, higher solids content black liquor has a higher viscosity which may cause operational problems in pumping and concentrating the liquor. Because of the benefits of increased steam production and lower fireside deposit formation rate, it is desirable to fire black liquor with the highest possible solids concentration. This upper limit of solids concentration is determined by allowable fume formation, pumpability of the liquor, and steam production rate limitations. The upper limit ranges between 63 and 80 wt % solids depending on the type and design of the recovery boiler.
The viscosity of black liquor can be reduced by heating in the absence of oxygen, which splits the lignin macromolecules contained in the liquor. U.S. Pat. No. 4,929,307 discloses a method for reducing viscosity by heating black liquor to 170.degree.-190.degree. C. and maintaining the liquor at this temperature for 1 to 60 minutes, preferably 1-5 minutes. U.S. Pat. No. 4,953,607 discloses a series of flash tanks and heat exchangers installed between the stages of a multiple effect evaporator wherein the liquor is heated indirectly to 190.degree.-200.degree. C. and held in a reactor vessel for 10-20 minutes to reduce the viscosity of the black liquor. The thermal stability and viscosity effects of temperature on black liquor are discussed in an article entitled "Thermal Stability of Kraft Black Liquor Viscosity at Elevated Temperatures" by J. D. Small et al in Ind. Eng. Chem Prod. Res Dev. 1985, 24, 608-614.
U.S. Pat. Nos. 4,239,589 and 4,313, 788 disclose the oxidation of black liquor in which high recovery of the heat of reaction is accomplished by integration with multiple effect evaporator stages. The extent of oxidation is controlled to oxidize the sodium sulfide in the liquor to sodium thiosulfate.
U.S. Pat. No. 4,718,978 discloses a method by which a portion of weak or partially concentrated black liquor is oxidized to a significant extent such that a substantial amount of the organic material is partially oxidized, and the oxidation is terminated just before the liquor becomes unpumpable. The oxidized liquor is blended with the remainder of the concentrated liquor and fed to the recovery boiler. The oxidation step significantly reduces the heat of combustion of the blended liquor.
Improved methods are needed to increase the efficiency of black liquor recovery and increase the capacity of recovery boilers in kraft pulp mills. In particular, methods are needed for handling the high solids liquor required for maximizing boiler capacity. The present disclosure and the claims which follow describe such an improved method.